Mike,
The reason for the latex error probably is as Jesper said; imaxima can't
find the breqn files. This should also be noted in the mylatex.log in
your temp file. It should say flexisym.sty not found and the same for
the other files. If you are using miktex, put the breqn files in
\texmf\tex directory. After this goto miktex options (should be in
start-menu, also texmf\miktex\bin\mo) and refresh the filename
database. After that you can run kpsewhich from the command line to make
sure that the files are found (for example 'kpsewhich flexisym.sty').
Osmo